1. Field of the Invention
The present invention relates to a superconductive Josephson device and a process for producing the same, more particularly to a Josephson device, such as a superconductive quantum interference device used as a sensor for measuring an extremely weak magnetic field, and a process for producing such a Josephson device.
2. Description of Prior Art
A superconductive quantum interference device includes a superconductive ring that couples with one or two Josephson junctions, and is used for measuring a magnetic field generated from the heart, eye ball, brain, and like organisms of the human body, and is abbreviated to a superconductive quantum interference device (hereafter reference to as SQUID).
The SQUID serves as a sensor for measuring such extremely weak magnetic fields as are generated from the organisms. However, production of the device including the Josephson junction is conducted with the use of a fine processing technique, and the performance as a sensor directly depends on the error in the shape of the device after processing. Further, superconductive material to be used is material which will be oxidized over time while being used. When the superconductive material in use is oxidized, it affects the performance of the device as a sensor as much as deformation of the Josephson junction. Therefore, it is difficult to maintain the initial performance of the device when produced over time.
Conventionally, a surface portion of the Josephson junction is oxidized in advance to protect the Josephson junction from the atmosphere, thereby forming an oxide film on the surface portion. However, in the case where oxide is formed on the surface portion, oxygen progressively diffuses over time in a thin film layer of superconducting material, for example Nb. As a result, there arises a problem in that a property of the device changes due to the reduced thickness of the Nb thin film.
The prior art covers the surface portion with synthetic resin material. In this prior art, a thermal expansion coefficient of the synthetic resin material differs greatly from that of the superconducting material. Accordingly, upon being subjected to a thermal hysteresis from liquid helium at a temperature of 4.2.degree. K. to ordinary temperature, minute cracks are caused in the synthetic resin material, whereby the SQUID is deteriorated.
In order to solve the foregoing problems and isolate the SQUID from the oxidated atmosphere, it can be considered to retain the SQUID in an atmosphere of inert gas, such as N.sub.2. However, this is not practical because it is difficult to prevent the leak.
Further, in the existing SQUIDs, it is difficult to form a uniform and exceedingly thin film of about 100 to 200 .ANG. in consideration of the current fine processing technique. Moreover, in the existing SQUIDs, it is extremely difficult to fabricate with minute length and width accurately and with satisfactory reproducibility in a processing which determines the size and shape of the junction. Therefore, the defect rate in production of the SQUID is extremely high.